This invention relates generally to plate-fin heat exchangers and more particularly to a counter-flow plate-fin heat exchanger with cross-flow headers used as a recuperator. Plate fin heat exchangers are typically monolithic structures created by brazing their many constituent pieces in a single furnace cycle. This general design presents several problems including the following:
1) A plate fin heat exchanger typically includes hundreds, if not thousands, of brazed joints. Thus, the overall quality of the finished product depends on the reliability of each and every brazed joint so that even one defective brazed joint can result in the entire heat exchanger being scrapped. As a result, assembly methods for plate fin heat exchangers are generally labor intensive as assemblers must avoid the creation of even a single poor braze among thousands in a typical heat exchanger.
2) The dimensions of the constituent parts used to assemble the heat exchanger must be maintained within close tolerances in order that differences in thickness do not compound into gross differences in load during the brazing cycle.
3) Edge bars or closure bars used to carry load through the edges of the heat exchanger make assembly both labor and material intensive and create stiffness and mass discontinuity differences in thermal response time.
With regard to the above design, counterflow plate-fin heat exchangers with cross-flow headers typically include a stack of headers sandwiched together to form an alternating gas/air/gas/air header pattern. Each pair of adjacent gas and air headers is separated by a relatively thin parting sheet. Additionally, conventional plate-fin heat exchangers incorporate edge bars or closure bars to seal about the perimeters of the parting sheets and prevent overboard leakage from the high pressure side of the heat exchanger. Inlet and outlet manifold ducts are welded transverse to the edge bars after the headers are assembled and brazed. The edge bars create a stiff and massive structural attachment between the parting sheets. Thermal loading produces faster thermal response in the lighter parting plates than the more massive edge bars. This difference in response time rate combined with the relative weakness of the parting plates can produce damage in the parting plates. Due to differences in the position and structural composition of the parting sheets and edge bars, the temperature changes do not affect the bars and sheets at the same rate. Since the parting sheets are structurally weaker than the edge bars, the parting sheets are strained.
A second problem associated with the use of edge bars in counterflow plate-fin heat exchangers is related to the sheet metal manifold ducts that are welded to the edge bars. The manifolds are welded to the stack of edge bars along the sides and corners of the core adjacent the header openings. Like the parting sheets, the manifold ducts respond quickly to changes in temperature. Since the edge bars do not respond to changes in temperature as quickly as the manifold ducts, the sheet metal experiences a shear load at or near the weld. As a result, the weld and the base metal in the heat affected zone is likely to become damaged.
U.S. Pat. No. 2,858,112 to Gerstung discloses a cross-flow heat exchanger for transferring heat from a liquid (FIG. 1) in which multiple pairs 10 of corrugated plates 12 and 14 are spaced apart by air centering means 16 and heat exchanger or edge bar elements 18 and 20. The edge bar elements 18 and 20 are sandwiched between the aligned header openings 30 and 32 of the respective plates 12 and 14. The utilization of the edge bar elements 18 and 20 adds undesirable rigidity and thermal mass discontinuity to the structure. As a result, the various layers of the structure are unable to move independently of one another during operation. Thus, the heat exchanger disclosed in the Gerstung patent is not appropriate for use with a gas turbine because the exchanger cannot withstand the tremendous temperature extremes produced by a gas turbine.
Great Britain Patent 1,304,692 to Lowery (FIGS. 1 and 5) discloses a cross-flow heat exchanger for transferring heat from a liquid including a plurality of metal plates 24 shaped and bonded together. The plates 24 have fin members 16 and 17 bonded to their respective outer surfaces. Each plate 24 has two centrally apertured raised end portions 25 and 26 and also has two parallel inverted channels 27 and 28. The respective units are assembled together by placing the next unit in the sequence with its raised end portions 25 and 26 in contact with equivalent raised end portions of the previous unit in the sequence, and by applying pressure to the juxtaposed pair of raised end portions 25 and 26. The relatively large intermeshing surface areas of adjacent raised end portions 25 and 26 results in the formation of rigid flow ducts so that the various layers of the final structure are incapable of moving and flexing relative to one another.
Based on the foregoing limitations known to exist in present plate-fin heat exchangers, it would be beneficial to provide a heat exchanger having a compliant bellows structure capable of elastically absorbing deflections produced by temperature gradients attendant with the heat exchange process and thermal gradients associated with installation or operation, so that the individual layers of the heat exchanger can move and flex freely relative to one another, and can accommodate thermal deflections throughout of plane deformation.